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Date: Wed, 7 Nov 2012 23:06:27 +0000
Subject: Re: Multi-dimensional list initialization
From: Oscar Benjamin <oscar.j.benjamin@gmail.com>
To: Joshua Landau <joshua.landau.ws@gmail.com>
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Cc: Python List <python-list@python.org>, Gregory Ewing <greg.ewing@canterbury.ac.nz>
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Xref: csiph.com comp.lang.python:32911

On 7 November 2012 22:16, Joshua Landau <joshua.landau.ws@gmail.com> wrote:
> On 7 November 2012 14:00, Oscar Benjamin <oscar.j.benjamin@gmail.com> wrote:
>> On 7 November 2012 13:39, Joshua Landau <joshua.landau.ws@gmail.com>
>> wrote:
>> > On 7 November 2012 11:11, Oscar Benjamin <oscar.j.benjamin@gmail.com>
>> > wrote:
>> >> A more modest addition for the limited case described in this thread
>> >> could
>> >> be to use exponentiation:
>> >>
>> >> >>> [0] ** (2, 3)
>> >> [[0, 0, 0], [0, 0, 0]]
>>
>> Exponentiation is expected to be asymmetric and is currently unused so
>> there is no ambiguity. The problem is if someone has already
>> subclassed list and added an exponentiation method.
>
> How is that a problem? They just wont get the functionality.

This is absolutely contrived but:
Library A defines a subclass of list that adds an exponentiation
operator thinking that it's okay to still use these objects as lists.
Library B has an API that expects a list and tries to use the list
copy-exponentiation on its input. A user passes a list type object
from library A into library B and hopefully gets an error but possibly
gets a subtle bug that is hard to track down.

It doesn't sound plausible to me but at least in principle there is a
backward compatibility problem.

> That said, losing:
> [0] * (2, 3) == [0] * [2, 3]
> would mean losing duck-typing in general. *Thus*, I fully agree with your
> choice of exponentiation.

Also there's no reason why tuples couldn't have the same
exponentiation operator (although for them it would be no different
from repeated multiplication).


Oscar